1. Field of the Invention
The present invention relates to a proton conductive solid electrolyte adapted for use in an electrode for a fuel cell or in a proton conductive membrane, to a membrane electrode assembly including a proton conductive solid electrolyte or a proton conductive membrane, and to a fuel cell comprising a membrane electrode assembly.
2. Description of the Related Art
Intensive research is being conducted on a proton conductive solid electrolyte for application in an electrochromic material and a sensor, particularly, in recent years in a fuel cell having a high energy density that is operated at low temperatures.
The fuel cell comprises a proton conductive film. A fuel electrode, which is also called an anode, is formed on one surface of the proton conductive film, and an oxidizing agent electrode, which is also called a cathode, is formed on the other surface of the proton conductive film. A fuel such as hydrogen or methanol is supplied to the fuel electrode (anode), and an oxidizing agent is supplied to the cathode. The fuel is electrochemically oxidized in the anode to form protons and electrons, which flow into an external circuit. The protons thus formed are transferred through the proton conductive film to reach the cathode, with the result that the oxidizing agent reacts with the electrons supplied from the external circuit to form water, thereby delivering electrical energy.
An ion exchange membrane formed of an organic polymer material containing perfluorosulfonic acid is known as the proton conductive film. To be more specific, the known proton conductive film includes, for example, an electrolyte containing a tetrafluoro ethylene-perfluoro vinyl ether copolymer as a base material and a sulfonic acid group as an ion exchange group. An example of this proton conductive film is a NAFION film manufactured by Dupont Inc. In the case of using the polymer material containing a perfluorosulfonic acid as an electrolyte, the water contained in the film is decreased by the drying to lower the proton conductivity. As a result, a severe water control is required in the case of using the electrolyte noted above at about 100° C. at which a high output can be obtained to make the system highly complex. Also, the polymer material containing perfluorosulfonic acid has a cluster structure, leading to a sparse molecular structure, with the result that a crossover phenomenon in which an organic liquid fuel such as methanol permeates through the electrolyte membrane to reach the cathode. Where the crossover phenomenon has been generated, the supplied liquid fuel reacts directly with the oxidizing agent, resulting in failure to deliver the generated electricity. It follows that a problem is brought about such that it is impossible to obtain a stable output.
A metal oxide that supports sulfuric acid and has a solid super acidity is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 2002-216537 as an inorganic acid-based ion exchange film. To be more specific, sulfuric acid is supported on the surface of an oxide containing at least one element selected from the group consisting of zirconium, titanium, iron, tin, silicon, aluminum, molybdenum and tungsten, and the oxide supporting the sulfuric acid is subjected to a heat treatment to immobilize the sulfuric acid on the surface of the oxide. In the metal oxide supporting sulfuric acid, the proton conductivity is produced by the immobilized sulfate group. However, since the sulfate group is decomposed by hydrolysis, the proton conductivity is lowered. It follows that the metal oxide supporting sulfuric acid is considered to be unstable when used in a fuel cell in which water is generated in the electricity generating process, particularly when used as a proton conductive film in a fuel cell using a liquid fuel.
Further, it is disclosed in Japanese Patent Disclosure No. 2003-142124 that a metal oxide hydrate exhibiting a proton conductivity is used as a proton conductive material. However, use of the metal oxide hydrate gives rise to the problem that, if the water of hydration is removed by the drying caused by the power generation under high temperatures, the construction of the metal oxide hydrate is shrunk, and the metal oxide is not brought back to the original form of the hydrate even if water is supplied later, resulting in failure to obtain adequate electricity generating performance.